Past regional cold and warm periods linked to natural climate drivers

Published: Friday, November 27, 2009, 16:44 [IST]

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Washington, November 27 (ANI): In a new research, a team of climate scientists has determined that intervals of regional warmth and cold in the past are linked to the El Nino phenomenon and the so-called "North Atlantic Oscillation" in the Northern hemisphere's jet stream.

These linkages may be important in assessing the regional effects of future climate change.

"Studying the past can potentially inform our understanding of what the future may hold," said Michael Mann, Professor of meteorology, Penn State.

Mann stresses that an understanding of how past natural changes have influenced phenomena such as El Nino, can perhaps help to resolve current disparities between state-of the-art climate models regarding how human-caused climate change may impact this key climate pattern.

Mann and his team used a network of diverse climate proxies such as tree ring samples, ice cores, coral and sediments to reconstruct spatial patterns of ocean and land surface temperature over the past 1500 years.

They found that the patterns of temperature change show dynamic connections to natural phenomena such as El Nino.

Mann and his colleagues reproduced the relatively cool interval from the 1400s to the 1800s known as the "Little Ice Age" and the relatively mild conditions of the 900s to 1300s sometimes termed the "Medieval Warm Period."

"However, these terms can be misleading," said Mann.

"Though the medieval period appears modestly warmer globally in comparison with the later centuries of the Little Ice Age, some key regions were in fact colder. For this reason, we prefer to use 'Medieval Climate Anomaly' to underscore that, while there were significant climate anomalies at the time, they were highly variable from region to region," he added.

The researchers found that 1,000 years ago, regions such as southern Greenland may have been as warm as today.

However, a very large area covering much of the tropical Pacific was unusually cold at the same time, suggesting that the cold La Nina phase of the El Nino phenomenon.

This regional cooling offset relative warmth in other locations, helping to explain previous observations that the globe and Northern hemisphere on average were not as warm as they are today.

Comparisons between the reconstructed temperature patterns and the results of theoretical climate model simulations suggest an important role for natural drivers of climate such as volcanoes and changes in solar output in explaining the past changes. (ANI)